The maximum frequency of signals in electrical and electronic devices has continued to increase in recent years. Devices that handle signals up to 100 GHz now exist. The increasing use of electro-optical devices, such as those used in optical communication and optical memory, has contributed to this increase in maximum frequency. Such advances in device performance have given rise to the need to provide a corresponding increase in the maximum frequency of apparatus used to observe and measure the waveforms of high-speed and high-frequency signal waveforms.
One way to observe the waveform of a high-speed or high-frequency electrical signal is to sample the electrical signal. Sampling the electrical signal converts the electrical signal to a lower-speed electrical signal that can be observed and measured conventionally. The broadest-band sampling oscilloscope currently commercially available has a bandwidth of only 50 GHz. Various approaches have been tried in an attempt to produce a sampling oscilloscope with a broader bandwidth. Some of the proposed approaches, and their drawbacks, will be described next.
One approach is described by K. Takeuchi and A. Mizuhara in Scanning Tunneling Optoelectronic Microscope with 2 Ps Time Resolution, 32 ELEC. LETT., 1709-1711 (1996 August). This approach uses a photoconductive switch as a sampling element. A photoconductive switch is controlled by light generated by a commercial pulsed light source having a full width at half maximum of about 100 fs. The electrical signal-under-test is sampled by passing it through the photoconductive switch. The photoconductive switch easily provides a response having a full width at half maximum of about 2 ps. A sampling circuit that uses such a photoconductive switch as its sampling element can respond with a time resolution of 2 ps, which corresponds to a frequency of about 175 GHz.
Photoconductive switches tend to have a dynamic switching characteristic that exhibits a tailing response in its turn-off characteristic, as will be described in more detail below. As a result of this, a sampling device that incorporates a photoconductive switch as its sampling element has frequency characteristics that are not flat at low frequencies, and the response of the sampling device gradually decreases with increasing frequency. Such characteristics are not desirable in a sampling device for high-frequency and high-speed electrical signals.
Another approach is described by Wesley C. Whitely et al. in 50 GHz Sampler Hybrid Utilizing a Small Shockline and an Internal SRD, AA-6 IEEE MTT-S DIGEST, 895-898 (1991). In this approach, a high-speed step signal is generated by a step recovery diode (SRD). The fall-time of the high-speed step signal is reduced by passing the signal through a non-linear transmission line (NLTL). After passing through the NLTL, the high-speed step signal is applied as a sampling pulse to a diode sampling bridge.
FIG. 1 is a block diagram of the sampling circuit 10 disclosed by Whitely et al. In the sampling circuit 10, the output of the local oscillator 12 is applied to a step recovery diode 14. The step signal generated by the step recovery diode is converted into a balanced step signal by the microstrip balun 16. The balanced step signal is fed by the coplanar line 18 to the non-linear transmission line 20. The non-linear transmission line is composed of a high-impedance transmission line with shunt varactor diodes placed at intervals along its length. The non-linear transmission line can be regarded as having N identical stages. Each stage contains a varactor diode centered in a length d of transmission line. An exemplary one of the varactor diodes is shown at 22. Passing the step signal through the NLTL decreases the fall time of step signal generated by the step-recovery diode 14.
The output of the NLTL 20 is connected by the coplanar lines 24, composed of the striplines 25 and 26, to the sampling signal inputs of the sampling chip 27. The step signal output by the NLTL propagates through the coplanar stripline 28, composed of the signal line 29 and the ground lines 30 and 31. After passing through the coplanar stripline 29, the step signal is reflected by the short-circuit at the input port 32 to form a sampling pulse.
The hold capacitor 40 is connected in series with the sampling diode 42, and the termination resistor 36 is connected in parallel with the series combination. The resulting series/parallel combination is connected between the signal line 29 and the junction of the strip line 25 and the ground line 30. The hold capacitor 44 is connected in series with the sampling diode 46, and the termination resistor 38 is connected in parallel with the series combination. The resulting series/parallel combination is connected between the signal line 29 and the junction of the strip line 26 and the ground line 31.
The signal-under-test SUT is received at the input port 32 and is fed to the connection point of the sampling diodes 42 and 46 by the signal line 29. The signal-under-test is sampled by the sampling diodes. The samples are held by the hold capacitors 40 and 44. The IF signal composed of the samples of the signal-under-test is coupled from the junction of the hold capacitor 40 and the sampling diode 42 and from the junction of the hold capacitor 44 and the sampling diode 46 via the hold resistors 48 and 50 and the low-pass circuitry 52 and 54. The IF signal is processed by a suitable IF amplifier (not shown).
The non-linear transmission line 20 is composed of at least ten stages each composed of a high impedance transmission line and a shunt varactor diode. The example described by Whitely et al. had 27 stages. The NLTL therefore imposes a large insertion loss on the high-speed step signal, which makes it difficult to generate large-amplitude sampling pulses for application to the sampling diodes 42 and 46. This causes the sampling device 10 to generate the IF signal with an insufficient signal-to-noise ratio for some applications. Moreover, the non-linear circuit elements included in the NLTL can cause multiple reflections in the NLTL. The reflected signals appear superimposed on the high-speed step signal and increase the rise- and fall-times of the sampling pulses. As a result, the narrowest sampling pulse achieved had a full width at half maximum of about 10 ps. This limits the maximum frequency of the signal-under-test that can be sampled by the sampling device 10.
A sampling device in which an air gap between the signal line and the substrate is introduced into the non-linear transmission line 20 is disclosed by S. T. Allen in Schottky Diode Integrated Circuits for Sub-Millimeter-Wave Application, PH.D. DISSERTATION, UNIVERSITY OF CALIFORNIA, SANTA BARBARA (1994 June). The air gap reduces the insertion loss and enables the sampling device to sample a signal-under-test with a maximum frequency as high as 725 GHz. Thus, the sampling device can sample a signal-under-test with a higher maximum frequency, but its complex structure results in a high manufacturing cost.
Accordingly, what is needed is a way to generate very short electrical sampling pulses with fast rise- and fall-times, and to generate such sampling pulses at low cost. What is also needed is to provide a sampling device that uses such sampling pulses.
The invention provides a sampling element that comprises a DC sampling voltage source, a threshold sampling circuit and a photoconductive switching element. The threshold sampling circuit includes a signal-under-test input, a sampling pulse input and an IF signal output, and has a sampling threshold with respect to sampling pulses received at the sampling pulse input. The photoconductive switching element is connected between the DC sampling voltage source and the sampling pulse input of the threshold sampling circuit.
The invention also provides a sampling device that comprises a DC sampling voltage source, a threshold sampling circuit, a photoconductive switching element and a light source. The threshold sampling circuit includes a signal-under-test input, a sampling pulse input and an IF signal output, and has a sampling threshold with respect to sampling pulses received at the sampling pulse input. The photoconductive switch is connected between the DC sampling voltage source and the sampling pulse input of the threshold sampling circuit. The light source is operable to generate optical pulses and is arranged to illuminate the photoconductive switching element with the optical pulses.
The sampling pulse input of the threshold sampling circuit may include a first sampling pulse input and a second sampling pulse input, the DC sampling voltage source may include a first output terminal and a second output terminal, and the photoconductive switching element may includes a first photoconductive switch and a second photoconductive switch. The first photoconductive switch is connected between the first output terminal and the first sampling pulse input, and the second photoconductive switch is connected between the second output terminal and the second sampling pulse input. In this embodiment, the DC sampling voltage source may include a first DC generator and a second DC generator, each of which as a positive output terminal and a negative output terminal. The positive and negative output terminals of the first DC generator are respectively connected to the first output terminal and to ground. The positive and negative output terminals of the second DC generator are respectively connected to ground and to the second output terminal.
The sampling pulse input of the threshold sampling circuit may include a first sampling pulse input and a second sampling pulse input, the photoconductive switching element may include no more than one photoconductive switch that comprises an input connected to the DC sampling voltage source and that additionally comprises an output, and the sampling element may additionally comprise a first stripline and a second stripline disposed adjacent and substantially parallel to one another. A first portion of the first stripline is connected to the output of the photoconductive switch. A second portion of the first stripline, different from the first portion, is connected to the first sampling pulse input. A portion of the second stripline is connected to the second sampling pulse input.
The photoconductive switching element has a dynamic switching characteristic that exhibits a tailing response below a tailing threshold, and the sampling threshold of the threshold sampling circuit is greater than the tailing threshold.
The threshold sampling circuit may include a threshold switching element having an intrinsic threshold voltage that defines the sampling threshold of the threshold sampling circuit and that is greater than the tailing threshold. The switching element may additionally comprise a reverse bias voltage source that is connected to the threshold sampling circuit and that operates to increase the sampling threshold of the threshold sampling circuit to a value greater than the intrinsic threshold voltage of the threshold switching element.
The sampling element may additionally comprise a reverse bias voltage source that is connected to the threshold sampling circuit and that operates to set the sampling threshold, at least in part, to be greater than the tailing threshold.
The sampling element may additionally comprise a controller connected to the DC sampling voltage source, a reverse bias voltage source and a controller connected to the reverse bias source. The controller connected to the DC sampling source operates to control the sampling voltage. The reverse bias source is connected to the threshold sampling circuit and operates to define the sampling threshold, at least in part. The controller connected to the reverse bias voltage source operates to control the reverse bias voltage source to set the sampling threshold to be greater than the tailing threshold.
The photoconductive switching element generates high-speed electrical pulses from the sampling voltage supplied by the DC sampling voltage. The high-speed electrical pulses are supplied as sampling pulses to the threshold sampling circuit. In response to short optical pulses generated by a laser, the photoconductive switching element generates sampling pulses potentially suitable for sampling high-frequency electrical signals. However, a photoconductive switching element typically has a dynamic switching characteristic that exhibits a tailing response. Such photoconductive switching elements are generally regarded in the art as being unsuitable for generating sampling pulses to control a sampling circuit because they do not allow the sampling circuit to turn completely OFF between successive sampling pulses. This impairs the accuracy of the samples generated by the sampling circuit.
The invention overcomes this shortcoming of photoconductive switching elements, and enables the substantial sampling pulse generating advantages of photoconductive switching elements to be realized by using a threshold sampling circuit as the sampling circuit. By setting the sampling threshold of the threshold sampling circuit to be greater than the tailing threshold of the sampling pulses generated by the photoconductive switching element, the ill effects of the tailing response on the sampling performance can be substantially eliminated.
The invention provides a sampling element, and a sampling device that incorporates the sampling element, that have a flat frequency response at low frequencies. Moreover, the sampling element and sampling device are capable of sampling a signal-under-test with a higher maximum frequency than a diode sampling bridge circuit driven by sampling pulses generated by a step-recovery diode. Moreover, the sampling element and sampling device according to the invention generate a higher-level IF signal that has a higher signal-to-noise ratio than can be achieved by a diode sampling bridge circuit driven by sampling pulses generated by a non-linear transmission line.
The sampling element and a sampling device according to the invention provide a low-cost way to measure a signal-under-test having a very high maximum frequency and to perform such measurements with a high time resolution, and with a flat frequency response.